1. Field of the Invention
This invention is directed to a working electrolyte for electrolytic capacitors. In particular, this invention is directed to the addition of silicate additives to the working electrolyte.
It is known that the electrical properties of electrolytic capacitors containing liquid working electrolytes degrade with inactivity. This degradation increases charging time, DC leakage, and decreases charge/discharge energy efficiency. The degradation is believed caused by hydration of the dielectric oxide by water present in the working electrolyte. Dielectric oxide hydration is a particularly critical issue for certain capacitor applications including, but not limited to, cardioverter defibrillators where charging time and charge/discharge energy efficiency are important.
2. Prior Art
Electrolytic capacitors are well known for use in a variety of electronic equipment such as consumer audio and video equipment, home appliances, power supplies, industrial electronics, military electronics, computers, telecommunication equipment, entertainment equipment, automotive devices, lighting ballasts, and medical devices. In general, electrolytic capacitors comprise an anode and a cathode segregated by at least one layer of separator material impregnated with electrolyte.
The anode is a valve metal. Examples of valve metals include, but are not limited to, tantalum, aluminum, titanium, and niobium. The valve metal is coated with a thin layer of the corresponding oxide that serves as a dielectric. The oxide is normally formed by a technique known as anodizing. The oxide film thickness is proportional to the anodizing voltage. As a result of the anodizing process, the desired oxide film thickness determines the capacitor operation voltage, operation temperature, and other performance requirements. For a given oxide dielectric and film thickness, the volumetric capacitance or energy density of the capacitor is a function of the specific surface area of the anode. To achieve high volumetric capacitance, porous anodes are used such as etched aluminum foils for an aluminum capacitor and pressed and sintered tantalum powder bodies for a tantalum capacitor.
The electrolyte in an electrolytic capacitor is a critical component. It determines the capacitor working voltage, equivalent series resistance (ESR), energy efficiency, operation temperature range, and the stability and reliability of the capacitor during shelf and operation life.
Electrolytic capacitors are the choice for high voltage high-energy pulse applications such as required for flash cameras and cardiac defibrillators. Their drawback is, however, degradation of electrical properties during periods of non-operation. Degradation increases charging time (build time) and DC leakage, and decreases the energy efficiency of the charge/discharge cycle after a period of non-operation.
Degradation after non-operation is believed caused by hydration of the dielectric oxide film from water in the working electrolyte. Hydration converts the dielectric oxide to a hydrous oxide that will not support a desired high electric field. This results in an increase in charging time (build time) and DC leakage. Increased charging time also means a decrease of charge/discharge energy efficiency (the energy delivered during discharging divided by the energy put in during charging). This behavior is important for applications such as implantable cardioverter defibrillators (ICD) where charging time is critical for patients' lives and charge/discharge energy efficiency affects device life and device volume.
The chemical composition of the working electrolyte affects the rate of oxide degradation. Since water is needed in the electrolyte for the purpose of oxide reformation and better electrical conductivity, hydration of the dielectric oxide due to attack by water cannot be avoided completely. Desirably, the water content in the electrolyte should be kept at a minimum. However, there is a tradeoff between oxide hydration and other capacitor properties. Electrolyte pH is also an important determinant for the rate of oxide hydration and has to be controlled in an optimal range.
To minimize oxide degradation and maintain device charging time, a common practice for ICDs is to “reform” the dielectric oxide by periodically charging the capacitor to or near the working voltage, followed by discharge through a non-therapy load or by shelf discharge. This maintains charging time, but it also consumes battery energy.
The present invention solves the problem of dielectric oxide hydration by using silicate additives in the working electrolyte for electrolytic capacitors.